1. Field of the Invention
The present invention relates to a method of controlling gear ratios (i.e., speed reduction ratios) of a continuously variable transmission in which transmission gear ratios are controlled based on a difference between an actual engine speed and a target engine speed to be set in response to an indicator representative of the driver's desire for acceleration or deceleration such as the amount of depression of the accelerator pedal, the opening of the engine throttle, or the like.
2. Description of the Related Arts
At the execution of such control of transmission gear ratios, the speed reduction ratio is so controlled that the actual engine speed is caused to approach the target engine speed. At that time, the magnitude of the rate of change in the transmission gear ratios, that is, gear shifting the speed must be appropriately set to obtain desired characteristics.
For this reason, there has been known a method of controlling gear ratios as disclosed in Japanese Patent Publication No. 59-44537 (Japanese Patent Laid-Open Publication No. 54-35558) in which the gear shifting speed is caused to change depending on the transmission gear ratio, and the larger the transmission gear ratio (=input engine speed/output engine speed), the more the gear shifting speed is increased. This aims to speed up the change in the transmission gear ratios to ensure a fully accelerating ability from a standstill.
Furthermore, Japanese Patent Laid-Open Publication No. 83-22737 discloses a method of controlling gear ratios in which the gear shifting speed is caused to change depending on the engine speed, and the higher the engine speed, the more the gear shifting speed is lowered. This aims to ensure a gear shifting response characteristic with respect to the operation of the accelerator pedal at a lower engine speed as well as realizing a gear shifting stability at a higher engine speed.
The above-described controlling device is constructed to perform a so-called kickdown gear ratio control by which the transmission gear ratio is increased (a shifting to lower gears takes place) when the target engine speed which is set in response to the depression of the accelerator pedal during the traveling is raised through the depression of the accelerator pedal.
The variation in engine speed, vehicular speed and the like which may be caused by this control will now be described with reference to a model shown in FIG. 11.
FIG. 11 diagrammatically shows a modelized automotive vehicle in which the output of an engine E is transmitted to wheels 120 by way of a power transmission system 100 including a continuously variable transmission.
Where an equivalent inertia weight of a rotator directly linked with an output shaft of the engine E, that is, of an engine system rotator is Ie, an output torque is Te, and an output revolution of the engine is Ne, the output torque Teo produced when the accelerator pedal is depressed to increase the engine speed (which is referred to as engine transient torque) is expressed as: EQU Teo=Te-Ie.times.dN/dt (1)
In other words, the increase or decrease in the engine speed necessitates a torque (Ie.times.dN/dt), and hence the actually obtained torque equals Teo.
The output of the engine E is transmitted through the power transmission system 100 to the output shaft (or the axle). Then, let a reduction gear ratio of the power transmission system 100 be i, a torque TDI transmitted to the output shaft is expressed in view of the expression (1) as: ##EQU1##
On the other hand, let the number of revolutions of the output shaft be ND, with a relationship of N=i.times.ND, the following expression is obtained by differentiating N with respect to time. EQU dN/dt=ND.times.di/dt+i.times.dND/dt (3)
Here, let an equivalent inertia weight of the vehicular body including its associated members be ID, the torque TD which will be actually transmitted to tires 120 from among the torque TDI transmitted to the output shaft as described above can be written as: EQU TD=TDI-ID.times.dND/dt (4)
It should be noted that the above equivalent inertia weight includes not only that of the vehicular body, but also the wheels and their associated rotators (for example, tires, a brake drum, an accelerator shaft, etc.) and a final reduction gear input shaft and its associated rotators (for example, a transmission counter shaft, gears on the output side, a propeller shaft, etc.).
This torque TD acts to drive the tires 120 to thereby accelerate the automotive vehicle. Accordingly, providing that a weight of the automotive vehicle is W, a running resistance of the automotive vehicle is fRL, and a rotational radius is r, then the acceleration .alpha. of the automotive vehicle is expressed as: EQU W=.alpha.=TD/r-fRL (5)
By substituting the above expression (4) for this expression (5), the following can be obtained. EQU W.times..alpha.=(TDI-ID.times.dND/dt)/r-fRL
Furthermore, when substituting the expressions (2) and (3) for the just above expression, the result is: EQU W.times..alpha.=I.times.Te/r-{(Ie.times.i.sup.2 /r+ID/r).times.dND/dt+Ie.times.i.times.di/dt.times.ND}-fRL
Here, since r.times.ND=V (vehicular speed) and r=dN/dt=.alpha.(acceleration), the just above expression can be expressed in the form EQU {W+(Ie.times.i.sup.2 +ID)/r.sup.2 }.alpha.=i.times.Te/r-(Ie.times.i.times.ND/r).times.di/dt-fRL(6)
On replacing respective constant terms with A, B, C, etc., the result is: EQU A.times..alpha.=B.times.Te-C.times.di/dt-D (7)
As is apparent from this expression (7), when effecting a kickdown gear shifting through the depression of the accelerator pedal, the engine torque Te is increased in accordance with the amount of the depression of the accelerator pedal. The acceleration is increased correspondingly to this increase of the engine torque, whereas the rate of change (di/dt) of the transmission gear ratio caused by the kickdown acts to decrease the acceleration .alpha..
For this reason, in case the rate of gear change is too high at the time of kickdown gear shifting, there occurs a negative acceleration .alpha.. In this case, the acceleration (G) to be applied onto the vehicular body and the engine speed (Ne) are varied as shown in, for example, FIGS. 9A and 9B, respectively. Thus, a negative acceleration (deceleration) is generated at the initial stage of kickdown gear shifting, which results in a sharp rise of engine speed.
However, the kickdown gear ratio control is performed with a view to increasing the transmission gear ratio so as to respond to the driver's desire for acceleration to thereby enhance the driving experience. Accordingly, it is not desirable in this case to generate such a deceleration even though it is a passing phenomenon.
To this end, the rate of gear change di/dt is generally to be set so that the acceleration .alpha. represented by expression (7) does not exhibit a negative value.
Such setting ensures that the acceleration (G) exerted on the vehicular body and the engine speed (Ne) are varied as shown in, for example, FIGS. 10A and 10B, respectively. Thus, engine speed is allowed to relatively smoothly rise without being subjected to any negative acceleration at the time of kickdown gear shifting.
In order to ensure a rapid response to the driver's desire for acceleration during the gear ratio control as described above, the rate of gear change di/dt is preferably set in such a manner that the acceleration .alpha. represented by expression (7) presents the largest value available within the positive range thereof. By virtue of such setting, at the kickdown gear shifting, the engine speed Ne can reach the target speed Neo in the shortest period of time without subjecting the vehicular body to any deceleration, thus realizing rapid kickdown gear ratio control.
Nevertheless, such setting of the rate of gear change may bring about an uncomfortable traveling feeling under circumstances when less variation in the traveling condition occurs as is often the case during constant-speed traveling on suburban roads or the like, because of a sensitive response of the engine speed relative to a slight action of the accelerator pedal.
For the purpose of avoiding such problem, it is conceivable to set the rate of gear change di/dt lower than the maximum of the acceleration .alpha. represented by expression (7) within the positive range thereof. However, in this case, under circumstances requiring a higher mobility, for example, when the vehicle is subjected to a lot of change in the traveling condition such as when traveling on mountainous or winding roads, the change in the engine speed is disadvantageously insensitive relative to the action of the accelerator pedal or the like, and hence the traveling condition having a higher mobility capable of responding to such variation in traveling condition is not to be excepted.
Also, when executing a kickdown gear shifting during the traveling at a higher engine speed, the same rate of gear change may bring about a larger gain in the engine speed as compared with the kickdown at the lower engine speed. For this reason, there arises a problem that the increase of the vehicular speed seems to the driver to be slow as compared with the rise in the engine speed, which may impart to the driver a feeling of idle running of tile engine.
Moreover, such kickdown gear ratio control is carried out with the rate of gear change which has been set as described above until the actual engine speed is correspondent with the target engine speed. Therefore, the engine speed is sharply increased before the actual engine speed approaches target engine speed, whereas the rate of rise in the engine speed is sharply lowered when the actual engine speed ultimately approaches the target engine speed.
Thus, at the time when reaching in tile intimate vicinity of the target engine speed, a kinetic energy hitherto being continuously applied to the inertia weight of the engine and the associated members during the sharp increase of the engine speed is directed to the inertia weight of the vehicular body and the associated members, to thereby bring about a rapid increase in the vehicular accelerations. Then, the driver may adversely feel such acceleration to be a shock. Under the circumstances having less variation in the traveling condition such as a constant-speed traveling conition on suburban roads, in particular, such acceleration may be felt as a shock more frequently by the driver, as compared with the circumstances requiring a higher mobility such as traveling on a mountainous roads, meandering roads or the like.
Also, in the case of the control in which the rate of the rise in the engine speed is sharply lowered when the actual engine speed approaches the target engine speed to thereby cause the actual engine speed to correspond to the target engine speed, there is often seen an overshoot condition where the actual engine speed exceeds the target engine speed. In case of the overshoot, it must be compensated for the recovery. However, this compensation may possibly lead to an undershoot. Furthermore, the occurrence of alternating overshoot and undershoot conditions may bring about a hunting in the revolution of the engine.